Co-fired multilayer circuit boards comprise two or more patterned conductor layers separated by a dielectric layer. The patterned conductor layers are connected by metallic conductors deposited in vias in the dielectric layer. The multiple layers are finally fired together to produce very dense circuit patterns.
Modern circuit boards must be able to accommodate complex integrated circuits requiring many input/output connections, and must be able to operate at fast switching speeds and high power levels. Circuit boards having alumina substrates have been developed for these purposes, but alumina requires high sintering temperatures which restrict the choice of conductors to refractory metals, such as tungsten, molybdenum and manganese, which have high electrical resistivities compared to other metals such as silver, copper or gold. Further, refractory metals such as molybdenum and tungsten require gold plating prior to soldering, adding to the cost of these circuit boards. Alumina also has a relatively high dielectric constant, which limits the signal speed. Moreover, alumina substrates have a high thermal coefficient of expansion which is incompatible with that of silicon, so that individual silicon chips or devices cannot form an integral part of the circuit.
Low temperature co-fired multilayer circuit boards made of either devitrifying or vitreous glasses are able to be fired at lower temperatures than alumina, so that other conductive metals can be used, for example silver conductors. Such circuit boards have many advantages for microelectronic packaging applications for the computer, telecommunications and military markets. They have lower shrinkages, low dielectric constants and the ability for direct attachment of silicon devices directly to the co-fired substrate.
However, for certain critical applications, rather stringent requirements are defined for low temperature co-fired circuit boards using silver conductors. They must have a thermal coefficient of expansion of 40-45.times.10.sup.-7 /.degree. C. from RT-400.degree. C.; a low dielectric constant at 1 MHz of about 6; resistance to silver migration; a dissipation factor at 1 MHz of less than 0.3%; be laser trimmable without chipping; have a breakdown voltage of more than 40 KV/mm; a specific resistivity of over 10.sup.14 ohm-cm; a flexure strength of about 2000 kg/cm.sup.-2 ; a high thermal conductivity of about 0.005 cal/cm-s-.degree. C. and be resistant to chemicals, particularly to acids used as etchants.
In addition, the low temperature co-fired circuit boards must have a firing temperature below about 900.degree. C.; a firing shrinkage of about 13-15% to ensure the production of dense, non-porous substrates; they must be able to be fired in air or nitrogen; and they must be able to form in excess of 15 laminate layers without blistering, delaminating or warping of the layers. Buried silver conductors in the substrates must have a conductivity of less than 6 milliohms/sq. Further, the use of silver conductors requires glasses in the dielectric layers resistant to silver diffusion, to avoid discoloration and staining. These circuit boards must also be low in cost, which is achieved by using silver conductors.
Present day glasses and ceramic circuit boards cannot meet all of these requirements.